1. Field of the Invention
The present invention relates to a process of determining a remaining life for a part, and more specifically to determining a remaining life for an airfoil used in the gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Industrial gas turbine engines (and even aero gas turbine engines) use several stages of blades and vanes in the turbine section of the engine that are under high temperature loads as well as mechanical loads. The airfoils include cooling air holes opening onto the outer surface of the airfoils for passing cooling air through and over the airfoil for cooling purposes. In an industrial gas turbine engine (IGT), the engine operates for a long period of time, or cycle, typically of around 40,000 hours before the engine is shut down for inspection and maintenance. Any premature shut down due to a problem like a damaged airfoil can be very costly due to the engine not performing its intended function like power generation. Because of the temperature, pressure and mechanical loads applied to the airfoil during operation, stresses occur. Also, residual stresses develop on the airfoil as the airfoil cools down to ambient temperature after shut-down of the engine. The residual stresses cause cracks to develop on the surface, typically starting at the cooling hole and extending outward thereof, when the airfoil cools down from its normal operating temperature. If the residual stresses on the airfoil extend into the wall of the airfoil fare enough, the cause can propagate through the wall of the airfoil, resulting in the airfoil to be non-useful.
The blades and vanes used in the turbine section of an industrial gas turbine engine are numerous and also very expensive. A typical IGT (industrial gas turbine) can have 50-100 blades or vanes in each stage, and have four stages, making the number of blades and vanes in the turbine in the order to 800 airfoils. The cost of a set of airfoils in the first stage can exceed one million dollars. Many times, an inspector would remove an airfoil that shows cracks to prevent a possible catastrophic event from occurring when the IGT is in operation. It is very costly to shut down an IGT prematurely. Therefore, an inspector may remove many airfoils that are still useful, even though many cracks appear on the surface. Thus, many used airfoils can accumulate that are not beyond the useful life in a gas turbine engine.
There is a need in the art of gas turbine engines to determine from cracks that appear in an airfoil if that airfoil is still capable of withstanding the operating conditions in the gas turbine engine for another engine operating cycle, or if the airfoil is at or near its useful lifetime.